Duashnoceras
Encyclopedia
Duashnoceras is an extinct ammonoid cephalopod genus included in the Stephanoceratidae that lived during the Bathonian
stage of the Middle Jurassic
Duashnoceras was established by Westermann in 1983, based on Stephanoceras floresi Burchhardt (1927) and originally placed in the Bathonian Zigzagiceratinae as a sugenus of Zigzagiceras but returned to the Stephanoceratidae by Westermann in 1984
Duashnoceras is a stephanoceratid of variable size; evolute with a very slight uncoiling of the last whorls. Juvenile whorls are depressed subtrapizoidal to subrectangular in section, with slightly convex flanks, arched venter, and well defined ventrolateral shoulders. With further growth the whorl section becomes more eliptical to subcircular with flanks more convex and without ventrolateral shoulders. Primary ribs, which form on the umbilical and dorso-lateral part of the whorls, appear almost immediately. At first the primary ribs are dense but become wide spread on the intermediary whorls, then dense again on the last whorls. Tubercles are conical, usually raised as spines on the intermediary whorls, and are independent of ribs which curve around them. Secondary ribs become thicker on the body chambers of microconchs but thinner of body chambers of macroconchs.
Microconch and macroconch variations, presumed male and female respectively, also differ in their apertures. The aperture of macroconch Duashnoceras is simple, a constriction followed by an elevated lip. That of microchonch Duashnoceras has a moderately long lappet, an extension on either side, sometimes preceded by a constriction.
The suture of Duashnoceras is intermediate between that of Stephanoceras
and Cadomites
. in coiling and whorl section Duashnoceras is similar to Stephanoceras, but differs in ornamentation.
Bathonian
In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...
stage of the Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....
Duashnoceras was established by Westermann in 1983, based on Stephanoceras floresi Burchhardt (1927) and originally placed in the Bathonian Zigzagiceratinae as a sugenus of Zigzagiceras but returned to the Stephanoceratidae by Westermann in 1984
Duashnoceras is a stephanoceratid of variable size; evolute with a very slight uncoiling of the last whorls. Juvenile whorls are depressed subtrapizoidal to subrectangular in section, with slightly convex flanks, arched venter, and well defined ventrolateral shoulders. With further growth the whorl section becomes more eliptical to subcircular with flanks more convex and without ventrolateral shoulders. Primary ribs, which form on the umbilical and dorso-lateral part of the whorls, appear almost immediately. At first the primary ribs are dense but become wide spread on the intermediary whorls, then dense again on the last whorls. Tubercles are conical, usually raised as spines on the intermediary whorls, and are independent of ribs which curve around them. Secondary ribs become thicker on the body chambers of microconchs but thinner of body chambers of macroconchs.
Microconch and macroconch variations, presumed male and female respectively, also differ in their apertures. The aperture of macroconch Duashnoceras is simple, a constriction followed by an elevated lip. That of microchonch Duashnoceras has a moderately long lappet, an extension on either side, sometimes preceded by a constriction.
The suture of Duashnoceras is intermediate between that of Stephanoceras
Stephanoceras
Stephanoceras is a Stephanoceratacean genus and type for the Stephanoceratidae, which lived during the Bajocian but is now extinct....
and Cadomites
Cadomites
Cadomites is an extinct ammonite genus from the superfamily Stephanocerataceae that lived during the Jurassic Period, which lasted from approximately 200 to 145 million years ago....
. in coiling and whorl section Duashnoceras is similar to Stephanoceras, but differs in ornamentation.